There is known from the prior art, in particular from CH Patent No. 580 358, an amplitude controlled quartz oscillator, shown in a simplified manner in FIG. 6. Such amplitude-controlled oscillators are used since they reduce the supply current as soon as the amplitude of the oscillator reaches a reasonable value. The oscillator core consists of an inverting amplifier with the quartz crystal as feedback and load capacitances C1 and C2 for providing the necessary phase difference. The transconductance necessary for the amplification stage, and thereby the current consumption of the oscillator, is for a given frequency proportional to the series resistance of the crystal, the oscillator amplitude and a capacitive factor that is a function of the load determined by C1 in series with C2. The oscillator core corresponding to the gain stage is formed by a single active transistor N1. The load is a current source I0 which is controlled by the amplitude regulator. The feedback resistance Rf which is necessary for polarising the gate of active transistor N1 has to be very high, so as not to load the resonator circuit. The complementary transistor P1 acts as polarisation current source for active transistor N1, via the current mirror that it forms with transistor P2. This gain stage easily allows amplitude to be controlled and requires low supply voltage. However, in order to ensure optimum selected transconductance, the supply current has to be two times higher than for a solution consisting of two active transistors disclosed within CH Patent No. 623 450 as shown in FIG. 7.
In the example of the prior art shown in FIG. 7, the gain stage is formed by two active transistors, a transistor N1 in series with a complementary transistor P1. The transconductances are added in this case. Insofar as the same continuous current passes through both transistors, half of the current is necessary in order to obtain the same transconductance with a current source load. Nonetheless, this solution requires a considerably higher supply voltage and the use of a high capacitance C in parallel with transistor P2, which powers the two active transistors, which occupies a large surface area on the circuit, and increases the cost thereof while greatly degrading the power supply rejection rate.